For efficient manufacturing of air bags, automatic cutting of webs of airbag fabric into desired shapes and sizes is required. In some apparatus for such automatic cutting, an automatic optical reading system, (herein also referred to as an automatic vision system), is used to read markings on the fabric to allow the apparatus to determine where to cut the fabric. The apparatus may cut the fabric using a cutting laser, for example. Such markings may comprise different weaving patterns, for example, utilising differently coloured yarns. U.S. Pat. No. 6,932,120 describes a marking method where groups of warp and groups of weft yarns are coloured to create patches, where they cross, that are readable by optically based guide systems controlling the cutting of the web, such patches forming datum markings or reference positions on the basis of which the automatic cutting equipment establishes where the fabric should be cut.
In such a known method for providing datum markings for recognition by optical reading and guiding means for cutting equipment using the intersections between groups of coloured (e.g. black) warp and weft yarns, the coloured warp yarns can be set up with the other warp yarns in the warp beam in the weaving loom, or alternatively, the coloured yarns can be fed from separate small creels. However, using coloured warp yarns in this way presents problems. Thus, each air bag design may require different positions of coloured warp yarns specific to the air bag geometry and thus different desired locations of the datum markings formed where the coloured weft yarns cross the coloured warp yarns.
Therefore, at every design change, the position of the coloured warp yarns may have to be changed, resulting in production down time.
Furthermore, discrimination by an automatic vision system between areas where groups of dark warps cross groups of dark wefts, on the one hand and between areas where groups of dark warps or wefts cross white wefts or warps respectively may not be entirely reliable and product may be lost as a result.
A number of special weave patterns have been developed in the past to locate datum or reference points for cutting without the use of contrasting colour yarns. These special weave patterns utilise different light reflection properties of different weaves to aid the vision system. Such arrangements, however, have proved unsuccessful for automated cutting systems. Existing optical detection systems are unable reliably to recognise such weave patterns consistently.
It is known to manufacture vehicle air bags by weaving a fabric in such a way that, over selected regions of the fabric, the fabric is woven in two layers with the warps and wefts of each layer being distinct from the warps and wefts of the other layer except in regions where the warps of both layers are interwoven with the same group of wefts and, conversely except in regions where the wefts of both layers are interwoven with the same groups of warps. The fabric thus woven is produced, as in any other commercial weaving process, as a continuous web and individual air bags are subsequently formed from the web by cutting the web along predetermined lines which are, for the most part, within the regions where the warps and wefts of both layers combine. Such air bags are herein referred to, for convenience as OPW air bags.